This invention relates to an overload protective device of an electric motor.
Heretofore, a heat responsive type overload relay has generally been used as the overload relay of an electric motor. Before explaining such overload relay a control circuit for driving a motor will first be described with reference to FIG. 1
In the control circuit shown in FIG. 1 the power of a source of alternating current 10 is rectified by a controlled rectifier 12 through a circuit breaker 11 and the rectified direct current is supplied to a direct current motor 13.
To protect the motor 13 against overload, according to a prior art system, the overcurrent condition is detected on the AC side by a current detector 14 such as a current transformer and the secondary current thereof is passed through a heat responsive type overload relay 15 for protecting the motor against a non-repeating overload. The term "non-repeating overload" is used to mean a permissible overload under a condition in which the load increases only once above a continuous rated load and then the load is reduced to zero as shown in FIG. 2. With this system, the motor can restart when its thermal characteristic returns to the original state.
Generally, however, in the actual running pattern, the motor runs under a load, although not always constant, after the occurrence of the overload so that in order to protect the motor operating under such load characteristic with the prior art system it is necessary to lower the setting of the relay 15 below the non-repeating overload of the motor shown by a solid line 1 in FIG. 3. However, as has been pointed out, since the actual load pattern is not always constant the selection of the setting of the relay is not simple but accompanied by the following problems. 1. When the setting is too high, the protection would become imperfect thus leading to burn out of the motor. 2. On the other hand, if the setting is too low the capacity of the motor would not be used completely.
A recent trend is to design the motor economically so that the latitude of the overload capacity becomes smaller. Hence more effective protective device has been desired.